When I connect to Wi-Fi and click ''Random Matchup'' then click ''Launcher Battle'' then click ''Free'' mode in Black (Even if my Pokemon are allowed, all are holding different items and are all different Pokemon and none holding Soul Dew), the sentence ''There was a problem with your Pokemon'' appears and then I'm sent back to the Pokemon Center... Why is this happening? Please help me if you can. Thanks!
The most probable reason is that your Pokemon are hacked. For example, if you had 5 legitimate Pokemon and 1 hacked, you will not be allowed to participate.
Check these basic things for hacked Pokemon:
Moveset: If they have weird moves, such as Roar of Time on Magikarp.
Place of origin: Like finding Mew on Route 1.
Stats: If a Magikarp had a 500 attack stat.
Level obtained: If Eevee was caught at LV100.
Poke Ball it was caught with: Like a Bulbasaur in an Ultra Ball.
Egg Obtained (If it hatched): Like if you got an egg from your rival
Egg Location Hatched: Most common would be hatched at ------.
Egg Level Hatched: Magikarp hatching at Level 100.
ID: There are certain IDs that are famous for hacked Pokemon.
OT: There are certain OTs that are famous for hacked Pokemon.
Ribbons: Hackers like to put every ribbon on a Pokemon, which is not possible.
Shininess: Some Pokemon cannot be shiny, such as Zekrom.
As Mew said, you can't connect to wi-fi if the pokemon are hacked, and believe me, I know.